Prisms In The Grass Ply Secrets From The Stars

Raindrops and prisms both are masters of light-bending. In a rainbow a beam of sunlight is bent as it enters the drop, reflects off the back of the drop and is bent a second time on exit, where it fans into a miniature rainbow. In a prism, light is bent at the front surface, starts to spread into individual colors which spread into a rainbow when they exit. Each color of light travels at a different speed through the water and glass, so each gets bent by a different amount, violet the most and red the least. This spreads the individual colors of white light apart to make a rainbow.  CC BY-SA 1.0

Prisms revolutionized astronomy in the late 19th and early 20th century and yet they’ve been under our noses since humankind witnessed its first frost. A prism separates white light into the colors of the rainbow. Many of us learned the order of those colors using the acronym Roy G. Biv, which stands for red, orange, yellow, green, blue, indigo and violet. Raindrops are probably the most familiar of nature’s prisms. Each drop works by refracting or bending white light from the sun and spreading it into a peacock’s tail of vivid hues.

Frost, made of hexagonal ice crystals, refract the morning sunlight like prisms, creating a shimmering carpet of color. I deliberately photographed the scene slightly out focus to better show what my eye saw.  A sharply focused image don’t show the color glints well. Bob King

Ice crystals make excellent prisms, too. On fall mornings after a clear night, we sometimes wake up to a scintillating carpet of frost on the front lawn. Frost forms when the surface temperature drops below that of the air above it. Water in the air condenses as liquid on grass, leaves and twigs, and then freezes to ice. If you could shrink yourself to the size of an atom you’d see water molecules arranging themselves into hexagonal (6-sided) rings one atop the other, building thousands of multi-story crystal palaces on a single blade of grass. And not just straight up but at every cock-eyed angle imaginable as water seeks to find purchase on the blade’s microscopic landscape.

Hexagons pillars of frost ice bud from two blades of grass in the front yard last Friday morning. Bob King

A couple mornings ago I stepped outside into a riot of color in my lawn. The night had been clear and the frost covered the grass and plants like a thick, silver mane. The crystals had grown large enough to act as prisms, and they refracted the chromatic glory out of the sun’s beams like I’ve rarely seen before. When I glanced across the grass, thousands of colorful sparkles twinkled across my field of vision. If I turned my head slightly the colors would shift. A yellow glint would shift to sapphire and a blue transform to ruby as my retina intercepted each of the different colors refracted by a multitude of minute ice crystals.

Lost in a forest of frost. Hexagonal frost ice on a fallen leaf seen up close. Note that some of the hexagons are irregular, not perfect. Bob King

Frost formed the next morning, too. But the crystals were much tinier and though they twinkled, color was lacking. I’ve seen these refractive effects on snow and even in beads of pine sap. I’ll never forget the afternoon I sat down along the shore of a nearby beaver pond and caught glints of multicolored sunlight refracted by a single bead of sap in a nearby spruce tree.

Astronomers direct a star’s light stream of light from a star of interest and direct it through a prism in a similar way only the light passes through a narrow slit first before it spreads into a horizontal  rainbow of color called a spectrum. The prism and slit are packaged in an instrument called a spectroscope. Careful examination of the spectrum will reveal narrow bands of color missing called Fraunhofer lines. Each line or pattern of lines represents a particular element or molecule in a star’s atmosphere that absorbs light radiated by the star’s surface. Think of them as fingerprints that reveal the identity of substance that robbed skinny strips of the rainbow.

A spectrum of the sun shows dark lines where elements like calcium and sodium in the sun’s atmosphere reveal themselves by absorbing sunlight from the blazing surface below. Wikipedia with additions by the author

Different substances absorb different colors. For instance, the element calcium absorbs narrow slices of violet light while sodium removes two hues of yellow-orange. In this way astronomers achieve what was once thought impossible — determining the composition of stars thousands of light years away from the comfort of their offices. Refract on that the next time frost whitens the green blades at sunrise.